1. Field of the Invention
This invention relates to methods and systems for generating an image of the radiation density of a source of photons located in an object.
2. Background Art
Parallel hole and pinhole are by far the most often used collimation methods for nuclear medicine applications, although fanbeam and conebeam collimators are also applied. These collimators have the advantage of being non-multiplexing and therefore every detected gamma ray defines a unique compact angular region, within which the gamma ray was generated or scattered. However, this uniqueness of information is achieved at the price of throwing away all other photons that do not fall into those possible paths defined by the collimator. Despite their drawback in sensitivity, these collimation methods are widely used in commercially available Gamma cameras.
Several mechanical collimators involving certain degrees of multiplexing have been studied in the past. These include the use of coded apertures, multiple pinholes and rotating slits, which allows a better detection sensitivity to be achieved. Unfortunately, this improvement is usually achieved at the expense of the amount of information conveyed by each detected photon. As a result, the signal-to-noise ratio in the collected data deteriorates dramatically when continuous background is introduced. In order to decode the data to provide a useful image, an unrealistically large number of counts are normally required. The uses of these image formation methods are currently limited to some particular imaging applications, in which the object-to-background ratio is reasonably large.
This trade-off is the same for Compton cameras. The information carried by the detected photons is diluted into a conical surface rather than a single ray. Furthermore, the angular accuracy achievable is also limited by the effect of Doppler broadening and electronic noise, especially at lower energies. It has been shown that at 140 keV, to compete with conventional gamma cameras the design of a Compton camera may require an unrealistic amount of semiconductor material for the scattering detector to provide enough detected scattering events.
In order to improve the imaging quality of multiplexing imaging systems, Wilson et al. proposed a so-called “synthetic collimator” in “A High Resolution Synthetic Collimator,” JOURNAL OF NUCLEAR MEDICINE, Vol. 39, p. 361, 1998. It is similar to a conventional multiple pinhole system, except the multiple pinhole aperture is moved between the object and detector during the data collection. As a result, data with different degrees of multiplexing is collected. It improves the condition number of the system response function to be inverted in the image reconstruction stage. It is shown that such an arrangement offers a superior reconstruction resolution when compared to a similar detector with a parallel hole collimator. The use of “synthetic collimation” also permits some three-dimensional information to be extracted. However, such a detector concept requires a detector with very good spatial resolution. The advantages of a “synthetic collimator” are also reduced dramatically when statistical noise is present in the data.
The idea of using combined mechanical and electronic collimation was proposed by Uritani et al. in “Electronically-Collimated Gamma Camera with a Parallel Plate Collimator for Tc-99 m Imaging,” IEEE TRANS. NUCL. SCI., Vol. 44, pp. 894-898, 1997. It reduces the effect of multiplexing while maintaining a relatively high open fraction on the collimator. In their detector, a multiple parallel plate collimator was placed in front of a Compton camera. The results showed an improved signal-to-noise ratio for imaging 99mTc tracer. However, the raw detection sensitivity is limited by the relatively low probability of detecting photons through Compton scattering effect.
Smith et al. have also reported a detector design with a similar concept in “Hybrid Collimation for Industrial Gamma-Ray Imaging: Combining Spatially Coded and Compton Aperture Data,” NUCL. INSTR. METH., A462, pp. 576-587, 2001. The design uses a coded aperture collimator in front of a Compton camera.
Other related articles include: J. W. LeBlanc et al., “C-SPRINT: A Prototype Compton Camera System for Low Energy Gamma Ray Imaging,” IEEE TRANS. NUCL. SCI., Vol. 45, pp. 943-949, 1998; S. R. Meikle et al., “An Investigation of Coded Aperture Imaging for Small Animal SPECT,” IEEE TRANS. NUCL. SCI., Vol. 48, pp. 816-821, 2001; and M. Singh et al., “An Electronically Collimated Gamma Camera for Single Photon Emission Computed Tomography, Part I: Theoretical Consideration and Design Criteria,” MED. PHYS., Vol. 10, pp. 421-427, 1983.
Related U.S. patents include: U.S. Pat. Nos. 4,506,374; 4,529,882; 5,462,056; 6,323,492; and 6,359,279.